This invention relates to an actuator for a memory storage device, such as a magnetic disc device, and more particularly to an actuator which enables an accurate positioning operation to be carried out easily.
In a magnetic disc device, a magnetic head is positioned at a high speed and with a high accuracy above a data track on a magnetic disc, which is rotated at a high speed, and the reading/writing of the data is done with respect to the track. A structure for moving this magnetic head is called an actuator. The actuator consists generally of an actuator assembly supporting the magnetic head and guiding the same in a predetermined direction, and a driving unit.
A conventional magnetic disc device, in which, for example, a rectilinearly driving actuator is used, is constructed so that the center of the driving force of the actuator and the center of gravity of the magnetic head-supporting actuator assembly are aligned with the direction in which the actuator assembly advances. Owing to such an arrangement, the whole of the driving force is used effectively as the actuator assembly-driving force in the conventional magnetic disc device, so that a smooth high-speed positioning operation can be carried out. In the case where the center of gravity of the actuator assembly and the center of the driving force are not aligned with each other, a rotary force occurs around the center of gravity of the actuator assembly. In this case, a part of the driving force turns into rotary moment force, so that the actuator assembly cannot be driven effectively. Moreover, since vibrations occur in the direction which is other than the driving direction, they remain as residual vibrations even after the positioning has been done, so that the positioning accuracy decreases.
In a rotary actuator, the effective rotational force can be obtained when the center of the rotation and that of an impact of the actuator assembly agree with each other but vibrations occur in the translational direction when these centers do not agree with each other. In the latter case, the positioning accuracy also decreases. It is known that the position of the center of an impact is determined depending upon the positional relation between the center of gravity of the actuator assembly and the center of the driving force in such a case as well. Under the circumstances, aligning the center of gravity of the actuator assembly with the center of the driving force with each other is done regularly during the designing of an actuator as disclosed in, for example, U.S. Pat. No. 4,415,941.
In the above-described conventional example, the center of gravity of the actuator assembly and the center of the driving force are aligned on the basis of the geometric arrangement of the actuator assembly. However, these centers are not aligned perfectly in practice due to uncertain facters, such as the mounting tolerance of each member, scatter of mass thereof and influence of frictional force thereof. In this case, the rotary torque working around the center of gravity of the actuator assembly can be expressed by the formula F.cndot..DELTA.r, wherein F is the driving force; and .DELTA.r the quantity of deviation, i.e., the rotary torque is proportional to the level of the driving force and the quantity of deviation.
In the magnetic disc devices developed in recent years, the level of the driving force continues to be increased in accordance with an increase in the rotational speed and accuracy thereof, while the level of a positioning error due to the residual vibration of the actuator assembly becomes unignorably high. Therefore, it is necessary that the center of gravity of the actuator assembly and the center of the driving force be aligned strictly with each other.